Transfer of solar irradiance through cirrus cloud layers

Reflection, transmission, and absorption of several cirrus cloud layers as functions of the solar zenith angle are obtained in the visible and near infrared of the solar spectrum by simplified radiative transfer calculations. The single-scattering computations for cirrus are carried out on the assumption that they contain randomly oriented long cylinders. The scattering parameters such as the phase function, the single-scattering albedo, and the extinction cross section are formulated and evaluated for incident wavelengths of 0.7, 2, 2.5, and 3 μ. A modified transfer method is developed with an approximation made only in the integral form of the radiative transfer equation. The method, which uses a small amount of computer time, yields an accuracy within 3% in most cases of conservative as well as nonconservative scattering. It is found that at 0.7 μ visible light a typical cirrus with an optical thickness of ∼2 shows a local albedo of ∼20% for zenith angles up to ∼60°. High values of transmission may be anticipated even if large absorption occurs in the clouds.